Metamaterials can give rise to a broad variety of anomalous transmission effects.
For instance, it is well known that, in spite of their inherent opacity, slabs of (homogeneous, isotropic)
epsilon-negative and mu-negative media can give rise to resonant tunneling phenomena
(with total transmission and zero phase delay) when paired as a bilayer.
Starting from this basic configuration, several extensions and generalizations can be investigated.

In [1],
in collaboration with
Nader Engheta (University of Pennsylvania)
and Andrea Alù (University of Texas at Austin),
we studied another counterintuitive resonant tunneling mechanism,
which entails pairing an epsilon-negative slab with a bilayer made of double-positive
(i.e., positive-permittivity and permeability) media. In particular, one of the two double-positive media
can be chosen arbitrarily (even vacuum), while the other may exhibit extreme (either near-zero or very high)
permittivity and permeability values. Our results on this counterintuitive tunneling phenomenon also
demonstrate the possibility of synthesizing double-positive slabs that effectively exhibit
epsilon/mu-negative-like wave-impedance properties within a moderately wide frequency range.
In [2],
we considered a different configuration featuring a homogeneous, isotropic epsilon-negative slab
paired with a uniaxially anisotropic double-positive slab, under obliquely incident illumination.
Unlike the Fabry–Perot-type resonant phenomena observed in the previous configuration (characterized
by standing waves in the double-positive layers and a nonzero phase delay),
the resonant phenomena in the proposed configuration are mediated by the excitation of localized surface modes at
the interface, are characterized by zero phase delay, and they depend on the slab thickness ratio (rather than sum), i
n a much closer analogy with what observed in the matched epsilon/mu-negative bilayers.
This allows us to establish a more direct and physically incisive analogy between the uniaxial-double-positive
slab and a matched (homogeneous, isotropic) epsilon/mu-negative slab, respectively.

Another intriguing phenomenon is the so called “extraordinary optical transmission”, i.e.,
the possibility of achieving strong enhancements in the transmission of electromagnetic
fields through arrays of sub-wavelength holes or slits in metallic screens
with respect to the very low levels predicted by the well-known Bethe’s theory.

In collaboration with Antonello Andreone’s Group (“Federico II” University of Naples),
we demonstrated (numerically and experimentally) the possibility of achieving substantial enhancements in the
transmission of transverse-electric-polarized electromagnetic fields through subwavelength slits
in a thin metallic screen by placing single or paired metallic cut-wire arrays at a close distance from the screen.
More specifically, in [3], we carried out a comprehensive numerical study,
which indicated that a richer phenomenology (involving both electric-and magnetic-type resonances)
could be attained by pairing a two cut-wire arrays at the two sides of the screen.
Subsequently, in [4], we reported on the first experimental evidence of
such extraordinary transmission phenomena, via microwave (X/Ku-band) measurements on printed-circuit-board prototypes.
The figure top panel schematizes the geometry of interes, whereas the center panel shows the fabricated prototype.
The bottom panel shows instead the microwave measurements.
A resonance peak is clearly visible around 14.4 GHz, with an intensity reaching nearly 80% of the full transmission,
i.e., about 910-times stronger than the response attainable in the absence of the cut-wire arrays.
Our results, in very good agreement with the full-wave numerical predictions, also
reveal a remarkable robustness of these phenomena with respect to fabrication tolerances and experimental
imperfections (misalignments, edge effects, etc.), and confirm the intriguing design potentials envisaged
in the previous numerical studies.

We show that resonant tunneling of electromagnetic fields can occur through a three-layer structure composed of a single-negative (i.e., either negative permittivity or negative permeability) slab paired with a bilayer made of double-positive (i.e., positive permittivity and permeability) media. In particular, one of the two double-positive media can be chosen arbitrarily (even vacuum), while the other may exhibit extreme (either near-zero or very high) permittivity and permeability values. Our results on this counterintuitive tunneling phenomenon also demonstrate the possibility of synthesizing double-positive slabs that effectively exhibit single-negative-like wave-impedance properties within a moderately wide frequency range.

We show that, under appropriate oblique-incidence and polarization conditions, the inherent opaqueness of a homogeneous, isotropic single-negative slab may be perfectly compensated (in the ideal lossless case) by a homogeneous, anisotropic (uniaxial) double-positive slab, so that complete tunneling (with total transmission and zero phase delay) occurs. We present an analytical and numerical study aimed at deriving the basic design rules, elucidating the underlying physical mechanisms, and exploring the role of the various involved parameters.

It has recently been shown that the transmission of electromagnetic fields through subwavelength slits (parallel to the electric field direction) in a thin metallic screen can be greatly enhanced by covering one side of the screen with a metallic cut-wire array laid on a dielectric layer. In this Letter, we show that a richer phenomenology (which involves both electric- and magnetic-type resonances) can be attained by pairing a second cut-wire array at the other side of the screen. Via a full-wave comprehensive parametric study, we illustrate the underlying mechanisms and explore the additional degrees of freedom endowed, as well as their possible implications in the engineering of enhanced transmission phenomena.

Recent numerical studies have demonstrated the possibility of achieving substantial enhancements in the transmission of transverse-electric-polarized electromagnetic fields through subwavelength slits in a thin metallic screen by placing single or paired metallic cut-wire arrays at a close distance from the screen. In this paper, we report on the first experimental evidence of such extraordinary transmission phenomena, via microwave (X/Ku-band) measurements on printed-circuit-board prototypes. Experimental results agree very well with full-wave numerical predictions, and indicate an intrinsic robustness of the enhanced transmission phenomena with respect to fabrication tolerances and experimental imperfections.